We propose to study the nutritional and metabolic capacity of the developing rat over the period when milk is the exclusive diet. The metabolism of glucose, the gluconeogenic precursors, lactate and alanine, the ketone bodies and the ketogenic precursors, free fatty acids and leucine, are to be investigated during the suckling period. Experiments are planned to establish the priority for utilization of these metabolites for respiration and for lipid synthesis at this time. It is proposed that lactate produced principally by the metabolism of glucose in brain and peripheral tissues is utilized by active gluconeogenesis in liver and kidney as a conservation mechanism for glucose homeostasis. The effect of different ketone body levels on glucose/lactate metabolism and on pyruvate metabolism to acetyl-CoA will be studied. Precursors which enhance ketone body production are to be used. The pattern of development of pyruvate dehydrogenase in organs during development will be determined in relation to the metabolic status of the animal to determine if its normal pattern can be suppressed or delayed depending on the metabolic status of ketone bodies or/and free fatty acids in blood. Metabolic and enzymic conditions which could simulate an energy deficit will be examined.